The present invention relates to a turbine inlet air-cooling system and, more particularly, to a turbine inlet air-cooling system using direct and indirect evaporative cooling to improve turbine performance and efficiency. The word “turbine” as used in this application should be interpreted to encompass a gas turbine or any other “air-breathing” engines, such as internal combustion engines (i.e., diesel engines) and fuel cells.
A limitation shared by all combustion turbines is that they are mass flow limited. The output power is limited by the mass of air passing through the bellmouth into the first stage of compression. The mass flow rate of air varies with ambient temperature due to changes in air density. A 50° F. increase in the ambient temperature typically causes more than 25% loss of power, as shown in FIG. 1. In addition, the temperature increase leads to increased fuel consumption and emissions of NOx. The problem thus is that gas turbine power decreases and heat rate increases with increasing ambient temperature.
One approach to avoiding the power degradation caused by high ambient temperature is to cool the inlet air. Various methods of doing this have been proposed, each with some degree of success, but also introducing new problems. The methods include vapor compression refrigeration, ice storage, absorption cooling, and evaporative cooling.
The traditional method of vapor compression refrigeration consumes about the same amount of fuel as that saved due to reduction in the heat rate, and the required added capital for the refrigeration equipment is comparable to the cost of the incremental turbine capacity increase. In addition, due to the phase out of CFCs, this approach relies on use of HCFCs and HFCs. HCFCs are currently in the process of being replaced by HFCs due to environmental concerns, and reliable, cost effective HFC alternatives are not yet fully developed. Cost and energy consumption may significantly increase due to utilization of an environmentally acceptable HFC. Thus, it is desirable to identify an alternative that is low in cost, consumes less energy, and does not rely on utilization of HFCs or HCFCs.
In recent turbine inlet air-cooling installations, the capital necessary for refrigeration equipment has been substantially reduced via thermal storage, e.g., using large store rooms of ice. In this approach, the refrigeration system is sized for only about 20% of the full refrigeration duty. When cooling inlet air, the ice storage supplies most or all of the refrigeration duty. The refrigeration system replenishes the ice during off-peak periods. This approach achieves increased turbine capacity, at a cost between $100/kW and $200/kW; however, its major disadvantages are that the cooling is available less than 20% of the time, and there is no net energy saving. Due to these constraints, it is viable only in conjunction with peaking combustion turbines for electric utilities. In addition, this approach relies on the use of HCFCs or HFCs, and shares the shortcomings with the traditional vapor compression system.
Use of absorption cooling to cool the turbine inlet air attracts interest because the heat content of the hot turbine exhaust can be used as the heat source for the absorption apparatus. This approach to inlet air-cooling has the advantages that a substantial degree of cooling is reliably available at all times, and the compression energy savings is almost fully preserved. However, the capital requirements for absorption refrigeration are typically substantially higher than for a mechanical compressor supplying the same cooling duty.
Evaporative cooling offers a low cost, low energy method for decreasing the compressor air inlet temperature. This approach relies on a significant difference between ambient wet bulb and dry bulb temperatures. Its major disadvantages are the unreliability of the cooling capacity due to the dependency upon vagaries of weather, and typically only a small amount of cooling is possible.